Serial-termination resistors for the ext memory

Document created by analog-archivist Employee on Feb 23, 2016
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A question comes up about serial-termination resistors for the ext memory. It
seems that in some eval-boards are resistors -in the range of several tens of
Ohm- in the address and/or data path. We will use SDRAM 133MHz + SRAM +
FerroRAM and of course Flash. We would like to have your suggestion whether
termination resistors are needed or not? for every memory or just SDRAM,
datapath and programmable lines...?

 

While we don't provide information specific to the EBIU lines, there are
general guidelines for line termination and other aspects of high frequency
design available in the hardware reference manual. Some signals are especially
critical for short trace length and usually require series termination (like
serial port pins).

Although typically not needed, designers may consider termination of SDRAM
clocks, control, address, and data to improve signal quality and reduce
unwanted EMI. As we all know, adding termination to fix a problem on an
existing board requires delays for new artwork and new boards. A transmission
line simulator is recommended for critical signals. IBIS models are available
from Analog Devices Inc. that will assist signal simulation software. Some
signals can be corrected with a small zero or 22 ohm resistor located near the
driver. The resistor value can be adjusted after measuring the signal at all
endpoints.

For details, see the reference sources in “Recommended Reading” in the System
Design chapter of the Hardware Reference manual for suggestions on transmission
line termination.

Having said this, we have seen a few customer's using series termination (low
value) on the Add/Data pins for the SDRAM. But there is no specific data
available as to what resistor value should be used. The exact value of
termination resistance depends upon the board layout and the input impedance of
the devices connected to the bus. As a safety major, you can keep a provision
for the termination (have 0 ohm resistors), so that later you can choose an
appropriate value.

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